The present invention pertains to the field of surface acoustic wave devices.
Surface acoustic wave (SAW) devices are microsonic devices which typically include a piezoelectric substrate and at least two comb-shaped conductive elements mounted on one surface of the substrate. Comb-shaped elements include teeth-like electrodes which are interleaved in alternated relationship to form an input transducer for launching acoustic signals along the surface of the substrate in response to an input electrical signal coupled to the comb-shaped elements. The acoustic signals are received by an output transducer constructed in a manner similar to the input transducer and converted into an output electrical signal. By selecting the amount of overlap between the two electrodes included in each pair of alternated electrodes and spacing between the pairs of alternated electrodes, a desired amplitude versus frequency response, such as, for example, the IF filter portion of a television receiver, may be obtained. SAW devices are desirable with respect to conventional filter circuitry constructed from discrete components because they are relatively small in size, readily manufactured by etching techniques and tend to be consistent from piece to piece.
One of the difficulties associated with SAW devices which have thus far kept them from being widely employed in television receivers and the like concerns the amplitude ratios of their out-of-band to in-band frequency responses which include undesired sidelobes of significant amplitudes compared with respect to the peak amplitude of the desired passband. In terms of the IF filter portion of a television receiver, this means that undesired adjacent channel signal components may have amplitude comparable with the amplitudes of signal components of the desired channel. SAW devices in which both input and output transducers are uniform, that is, transducers in which the amount of overlap of the two electrodes in each pair of alternated electrodes is substantially the same throughout the transducer, tend to have amplitude versus frequency responses in which the ratio of the largest sidelobe amplitude and the largest passband amplitude is below -26.4 dB (decibels) and therefore not particularly suitable for the IF portion of a television receiver. On the other hand, SAW devices in which at least either the input or output transducer is nonuniform, that is, a transducer which the overlap of the two electrodes in each electrode pair is not substantially uniform throughout the transducer, are capable of providing amplitude versus frequency responses suitable for the IF portion of a television receiver in which the ratio between the largest sidelobe amplitude and the largest passband amplitude exceeds -40 dB. Unfortunately, because the achievement of a desired amplitude versus frequency comes at the expense of added numbers of pairs of alternate electrodes, nonuniform transducers usually require many more pairs of alternated electrodes than do uniform transducers, therefore, nonuniform transducers are relatively large and expensive compared with uniform transducers. Furthermore, because the average amount of overlap of all the pairs of alternated electrodes of nonuniform transducers tends to be less than that of uniform transducers and the average amount of overlap of a transducer is related to its energy conversion efficiency, nonuniform transducers tend to be less efficient than do uniform transducers. In addition, because the amplitude of a surface acoustic wave launched by a nonuniform transducer is not constant across its wave front, the combined amplitude versus frequency response of a nonuniform transducer and another nonuniform transducer will in general not be the product of the two respective transducers.
While transducer configurations have been suggested for improving the frequency selectivity of SAW devices (see, for example, U.S. Pat. Nos. 3,801,937 and 4,006,438) these configurations exhibit one or more of the disadvantages set forth above. Thus, a transducer for use in the IF portion of a television receiver and the like should have the frequency selectivity characteristics of nonuniform transducers but the size, cost and energy conversion characteristics of a uniform transducer.